Diesel engines produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). A variety of emissions control devices exist for treating one or more of each type of pollutant. These emissions control devices are often combined as part of an exhaust system to ensure that all four classes of pollutant are treated before emission of the exhaust gas into the environment.
Diesel engines, particularly heavy duty (HD) diesel engines, are being designed to have improved fuel economy. As a consequence of these designs, the diesel engines output higher levels of oxides of nitrogen (NOx) and the exhaust systems for such engines are required to provide increasingly higher NOx conversion to meet emission regulations.
Selective catalytic reduction (SCR) has been demonstrated to be an effective solution for meeting NOx emission requirements and regulations for diesel engines, particularly HD diesel engines. With increasing demand for improved fuel economy, improving the NOx emission reduction of the SCR system would allow for higher engine out NOx emissions. However, such requirements are challenging for current exhaust system designs, which typically consist of a diesel oxidation catalyst (DOC), a catalyzed soot filter (CSF) and an SCR catalyst.
One approach is to replace the CSF with a diesel particulate filter (DPF) coated with an SCR catalytic composition while keeping the flow-through SCR catalyst downstream. An example of such an exhaust system is described in SAE 2014-01-1525. Selective catalytic reduction catalysts on a filter substrate have been shown to have high NOx conversion capabilities (see SAE 2008-01-0072, SAE 2011-01-1312 and SAE 2012-01-0843).
It is also desirable to minimise the amount of nitrous oxide (N2O) in exhaust gas emissions. The US Environmental Protection Agency has stated that the impact of 1 pound of nitrous oxide (N2O) in warming the atmosphere is over 300 times that of 1 pound of carbon dioxide (CO2). Nitrous oxide (N2O) is also an ozone-depleting substance (ODS). It has been estimated that nitrous oxide (N2O) molecules stay in the atmosphere for about 120 years before being removed or destroyed. Current legislation for regulating engine emissions does not limit nitrous oxide (N2O) because it is regulated separately as a greenhouse gas (GHG).